Economic cruise control

ABSTRACT

Economic cruise control where a response of a cruise control system used to control a powertrain is determined based at least in part on whether a driver of a vehicle system has selected to manage cruise control according to performance or efficient cruise control strategies. The performance strategy corresponding with fulfilling a request to accelerate or otherwise increase energy demands less efficiently, and in some cases more quickly, than if the request were to be fulfilled with the efficient cruise control strategy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/456,590 filed Apr. 26, 2012, now U.S. Pat. No. 8,666,577, thedisclosure of which is hereby incorporated in its entirety by referenceherein.

TECHNICAL FIELD

The present invention relates to vehicle cruise control of the typewhere a driver selects a desired vehicle speed and a vehicle controllerattempts to automatically maintain the selected vehicle speed throughcorresponding control of a vehicle powertrain.

BACKGROUND

During operation of a vehicle, including a hybrid electric vehicle(HEV), plug-in hybrid electric vehicle (PHEV) and battery electricvehicle (BEV), when cruise control is engaged and the driver decides toaccelerate, the vehicle may respond with a downshift and associatedhigher engine RPM to provide commanded acceleration. This can provide arelatively fast response but it may also be inefficient with respect tofuel economy. Drivers of electric or partially electric vehicles, inparticular, tend to desire energy efficiency over performance, at leastin some respects. The present invention contemplates allowing the driverto specify speed change preferences when operating in cruise control. Itis believed that that this will enhance satisfaction of drivers thattend to desire efficiency over performance, at least during someoperating conditions.

SUMMARY

One non-limiting aspect of the present invention relates to a method ofcontrolling a powertrain of a vehicle comprising: determining ancruise-controlled acceleration request; determining a first accelerationaction of the powertrain to meet the cruise-controlled accelerationrequest according to a first assist strategy; determining a secondacceleration action of the powertrain to meet the cruise-controlledacceleration request according to a second assist strategy, the secondassist strategy being slower and more efficient than the first assiststrategy; determining driver selection of the first or second assiststrategy; and controlling the powertrain to meet the cruise-controlledacceleration request according to the first acceleration action in theevent the first assist strategy is selected and according to the secondacceleration action in the event the second assist strategy is selected.

Embodiments include a method for controlling a vehicle having a batterypowered traction motor comprising receiving a signal associated with adriver requested cruise control mode, accelerating the vehicle at aslower rate to attain a cruise control target speed when operating in acruise control economy mode, and accelerating the vehicle at a fasterrate to attain a cruise control target speed when operating in a cruisecontrol performance mode. In one embodiment the method includesaccelerating the vehicle at the slower rate by maintaining a currenttransmission gear ratio while accelerating the vehicle. In oneembodiment, the vehicle includes an internal combustion engine poweringthe vehicle in the cruise control mode and the method includesaccelerating the vehicle at the slower rate by increasing wheel torqueby operating the battery powered traction motor to assist the internalcombustion engine.

Various embodiments according to the present invention may includeswitching from the slower rate to the faster rate before attaining thecruise control target speed in response to an override event.Representative override events may include a signal generated inresponse to a cruise control request button being activated for apredefined period of time, and vehicle acceleration being below acorresponding threshold, for example.

In one embodiment, the vehicle includes an internal combustion enginepowering the vehicle in the cruise control mode and accelerating thevehicle at the faster rate comprises downshifting a vehicletransmission.

Various embodiments according to the present invention may include amethod of controlling a powertrain of a hybrid vehicle having aninternal combustion engine, an electrically powered traction motor, anda transmission that includes controlling the engine, the traction motor,and the transmission in response to a cruise control accelerationrequest to selectively inhibit transmission downshifts for a currentvehicle speed based on a selected one of a plurality of operating modes.The plurality of operating modes may include an efficiency mode, whereinthe transmission downshifts are inhibited when the efficiency mode isselected. In embodiments where the plurality of operating modes includesan efficiency mode, controlling the engine, the traction motor, and thetransmission may include accelerating the vehicle without increasingengine rotational speed when operating in the efficiency mode. Inaddition, the method may include accelerating the vehicle using thetraction motor before using the engine or downshifting the transmission.In one embodiment, controlling the engine, the traction motor, and thetransmission comprises accelerating the vehicle using electric energyprovided by one of a high voltage battery and a fuel cell.

Non-limiting aspects of the invention may also include switching betweenthe plurality of operating modes to increase acceleration in response toan override event, which may correspond to vehicle acceleration beingbelow a corresponding threshold, for example. Other non-limiting aspectsof the invention may include accelerating the vehicle using the enginewhen operating in an efficiency mode and stored electric energy forpowering the traction motor is less than a corresponding threshold.

Various embodiments of the present invention include a vehicle having anelectrically powered traction motor, a transmission, and a controllercoupled to the traction motor and the transmission, the controlleraccelerating the vehicle at a slower rate to attain a cruise controltarget speed when operating in a first cruise control mode, andaccelerating the vehicle at a faster rate to attain a cruise controltarget speed when operating in a second cruise control mode. The vehiclemay also include an internal combustion engine coupled to thecontroller, wherein the controller accelerates the vehicle using theinternal combustion engine when operating in the second cruise controlmode. In one embodiment, the controller switches between the firstcruise control mode and the second cruise control mode in response to acruise control acceleration request when vehicle acceleration is below acorresponding threshold for a predetermined period of time. In oneembodiment, the controller inhibits transmission downshifts whenoperating in the first cruise control mode. Embodiments include avehicle having an internal combustion engine communicating with thecontroller, wherein the controller increases torque provided by thetraction motor before increasing torque provided by the engine inresponse to a cruise control acceleration request.

One non-limiting aspect of the present invention relates to the firstacceleration action downshifting the powertrain from a current gear to alower gear and the second acceleration action maintains the currentgear, thereby causing the second assist strategy to be slower and moreefficient than the first assist strategy.

One non-limiting aspect of the present invention relates to the firstacceleration action accelerating with assistance from an internalcombustion engine and the second acceleration action includesaccelerating solely with assistance from an electric motor, therebycausing the second assist strategy to be slower and more efficient thanthe first assist strategy.

One non-limiting aspect of the present invention relates to switchingfrom the second assist strategy to the first assist strategy afterbeginning the second acceleration action and prior to satisfying thecruise-controlled acceleration request in the event an override event isdetermined.

One non-limiting aspect of the present invention relates to determiningthe override event in the event the driver depresses a cruise controllerrequest button for a predefined period of time.

One non-limiting aspect of the present invention relates to determiningthe override event in the event acceleration after beginning the secondacceleration action is slower than a predefined threshold.

One non-limiting aspect of the present invention relates to determiningthe override event in the event vehicle speed fails to increase afterbeginning the second acceleration action.

One non-limiting aspect of the present invention relates to method ofcontrolling a powertrain of a vehicle comprising: determining a requestfor cruise-controlled acceleration; determining a first strategy for thepowertrain to meet the request; determining a second strategy for thepowertrain to meet the request that is more efficient than the firststrategy; and controlling the powertrain according to a selected one ofthe first and second strategies.

One non-limiting aspect of the present invention relates to the firststrategy shifting the powertrain from a current gear and the secondstrategy maintains the current gear.

One non-limiting aspect of the present invention relates to the firststrategy accelerating with assistance from an internal combustion engineand the second strategy accelerating without assistance from theinternal combustion engine.

One non-limiting aspect of the present invention relates to the secondstrategy accelerating with assistance from an electric motor.

One non-limiting aspect of the present invention relates to the secondstrategy accelerating with electric energy provided from one of a highvoltage battery and fuel cell.

One non-limiting aspect of the present invention relates to switchingfrom the second strategy to the first strategy in the event an overrideevent is determined.

One non-limiting aspect of the present invention relates to the firststrategy satisfying the request in less time than the second strategywould take to satisfy the request.

One non-limiting aspect of the present invention relates to selectingthe first strategy in the event energy available from an electricstorage source used to power acceleration of the powertrain is less thana predefined threshold.

One non-limiting aspect of the present invention relates to method ofcontrolling a powertrain of a vehicle comprising: determining a requestto increase vehicle speed using cruise control; determining a firststrategy for the powertrain to meet the request; determining a secondstrategy for the powertrain to meet the request; and controlling thepowertrain according to a driver selected one of the first and secondstrategies.

One non-limiting aspect of the present invention relates to the secondstrategy being more efficient than the first strategy.

One non-limiting aspect of the present invention relates to determiningthe driver selected one of the first and second strategies according toa driver input received in response to an inquiry requesting the driverto select between performance and efficient cruise control, the firststrategy being selected in the event performance cruise control is thedriver input and the second strategy being selected in the eventefficient cruise control is the driver input.

One non-limiting aspect of the present invention relates to the firststrategy downshifting the powertrain from a current gear and the secondstrategy maintains the current gear.

One non-limiting aspect of the present invention relates to the firststrategy accelerating with assistance from an internal combustion engineand the second strategy includes accelerating without assistance fromthe internal combustion engine.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is pointed out with particularity in the appendedclaims. However, other features of the present invention will becomemore apparent and the present invention will be best understood byreferring to the following detailed description in conjunction with theaccompany drawings in which:

FIG. 1 schematic represents a vehicle controlled in accordance with onenon-limiting aspect of the present invention.

FIG. 2 illustrates a flowchart of a method for economic cruise controlin accordance with one non-limiting aspect of the present invention.

FIG. 3 illustrates a graph of operating strategies for the vehicle whenassistance from the electric motor is unavailable in accordance with onenon-limiting aspect of the present invention.

FIG. 4 illustrates a graph of operating strategies for the vehicle whenthe electric motor is available in accordance with one non-limitingaspect of the present invention.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

Referring now to the drawings, FIG. 1 is a simplified, exemplaryschematic representation of a vehicle 10 controlled in accordance withone non-limiting aspect of the present invention. The vehicle 10 mayinclude an engine, such as an internal combustion engine (ICE) 12 and/oran electric machine, or generator 14. The engine 12 and the generator 14may be connected through a power transfer arrangement, which forexemplary purposes is shown to be a planetary gear arrangement 16, asthe present invention fully contemplates to use or incorporation ofother types of power transfer arrangements, including but not limited toother gear sets and transmissions that connect the engine 12 to thegenerator 14. The planetary gear arrangement 16 is shown to include aring gear 18, a carrier 20, planet gears 22, and a sun gear 24.

The generator 14 may be operable to output torque to a shaft 26connected to the sun gear 24, in place or in additional to the engine12, which may output torque to a crankshaft 28 connected to a shaft 30through a passive clutch 32. The clutch 32 may provide protectionagainst over-torque conditions. The shaft 30 may be connected to thecarrier 20 of the planetary gear arrangement 16, and the ring gear 18may be connected to a shaft 34, which may be connected to a first set ofvehicle drive wheels, or primary drive wheels 36, through a gear set 38.

The vehicle 10 may include a second electric machine, or traction motor40, which can be used to output torque to a shaft 42 connected to thegear set 38. Other vehicles within the scope of the present applicationmay have different electric machine arrangements, such as more or fewerthan two electric machines. The electric machine arrangement shown inFIG. 1 (i.e., the motor 40 and the generator 14) can both be used asmotors to output torque. Alternatively, each can also be used as agenerator, outputting electrical power to a high voltage bus 44 and toan energy storage system 46, which may include a battery 48 and abattery control module (BCM) 50.

The battery 48 may be a high voltage battery that is capable ofoutputting electrical power to operate the motor 40 and the generator14. The BCM 50 may act as a controller for the battery 48. Other typesof energy storage systems can be used with a vehicle, such as thevehicle 10. For example, a device such as a capacitor can be used,which, like a high voltage battery, is capable of both storing andoutputting electrical energy. Alternatively, a device such as a fuelcell may be used in conjunction with a battery and/or capacitor toprovide electrical power for the vehicle 10.

As shown in FIG. 1, the motor 40, the generator 14, the planetary geararrangement 16, and a portion of the second gear set 38 may generally bereferred to as a transmission 52. To control the engine 12 andcomponents of the transmission 52 (i.e., the generator 14 and motor 40)a vehicle control system, shown generally as vehicle controller 54, maybe provided. Although it is shown as a single controller, it may includemultiple controllers that may be used to control multiple vehiclesystems. For example, the controller 54 may be a vehicle systemcontroller/powertrain control module (VSC/PCM). In this regard, the PCMportion of the VSC/PCM may be software embedded within the VSC/PCM, orit can be a separate hardware device.

A controller area network (CAN) 56 may allow the controller 54 tocommunicate with the transmission 52 and the BCM 50. Just as the battery48 includes a BCM 50, other devices controlled by the controller 54 mayhave their own controllers. For example, an engine control unit (ECU)may communicate with the controller 54 and may perform control functionson the engine 12. In addition, the transmission 52 may include atransmission control module (TCM), configured to coordinate control ofspecific components within the transmission 52, such as the generator 14and/or the motor 40. Some or all of these various controllers can makeup a control system in accordance with the present application. Althoughillustrated and described in the context of the vehicle 10, which is aHEV, it is understood that embodiments of the present application may beimplemented on other types of vehicles, such as those powered by aninternal combustion engine alone, electric motor alone or a fuel cell.

Also shown in FIG. 1 are simplified schematic representations of abraking system 58, an accelerator pedal 60, and an air conditioningsystem 62. The braking system 58 may include such things as a brakepedal, position sensors, pressure sensors, or some combination of thetwo, as well as a mechanical connection to the vehicle wheels, such asthe wheels 36, to effect friction braking. The braking system 58 mayalso include a regenerative braking system, wherein braking energy iscaptured and stored as electrical energy in the battery 48. Similarly,the accelerator pedal 60 may include one or more sensors, which, likethe sensors in the braking system 58, may communicate with thecontroller 54. The air conditioning system 62 may also communicate withthe controller 54. The on/off status of the air conditioning system canbe communicated to the controller 54, and can be based on, for example,the status of an operator actuated switch, or the automatic control ofthe air conditioning system 62 based on related functions such as windowdefrost.

The vehicle 10 may include an information display system 64 operable toprovide relevant vehicle content to the driver of the vehicle 10. Theinformation display system may include the controller 54 and aninformation display 66. The information display system 64 may alsoinclude its own control system operable to communicate with thecontroller 54 and to perform control functions on the informationdisplay 66, although the controller 54 may also function as theinformation display's control system. The controller 54 may beconfigured to receive input that relates to current operating conditionsof the vehicle 10, and the controller 54 may provide outputs such thatthe information display 66 conveys driving efficiency information orother information relating to the operation of the vehicle 10 to thedriver.

The information display 66 may be disposed within a dashboard (notshown) of the vehicle 10, such as in an instrument panel or centerconsole area. Moreover, the information display 66 may be part ofanother display system, such as a navigation display system, or may bepart of a dedicated information display system. The information display66 may be a liquid crystal display (LCD), a plasma display, an organiclight emitting display (OLED), or any other suitable display. Theinformation display 66 may include a touch screen for receiving driverinput associated with selected areas of the information display 66. Theinformation display system 64 may also include one or more buttons (notshown), including hard keys or soft keys, located adjacent theinformation display 66 for effectuating driver input. Driver input mayinclude selection of a vehicle operating mode, such as a performancemode or an efficiency mode, for example. Other operator inputs known toone of ordinary skill in the art may also be employed without departingfrom the scope of the present application.

The vehicle may include a cruise control system 68 operable to control,direct, and/or request control, such as with assistance from thecontroller 54, to engage and otherwise control operation of the ICE,motor, and powertrain, including but not limited to controlling shiftingoperations of the powertrain/transmission 52 according to a desiredoperational strategy. One non-limiting aspect of the present inventioncontemplates the cruise control system 68 being operable to control thevehicle in cruise control mode where a desired vehicle speed isautomatically maintained, e.g., without continuous driver interactionand/or manipulation of the accelerator pedal 60. The cruise controlsystem 68 may include a cruise control interface 70 operable to set thedesired vehicle speed and receive other inputs from the user associatedwith performing cruise control related functions, or in some cases aremote or wireless entity operable to control the vehicle.

A cruise control function contemplated by one non-limiting aspect of thepresent invention relates to cruise-controlled acceleration, which isgenerally defined to correspond with any type of vehicle event where thevehicle 10 is instructed while operating in cruise control mode toaccelerate, change speed, or otherwise engage in some type of actionthat requires increased output from the engine and/or motor and/orincreased consumption of energy, such as fuel from a fuel tank, fuelcell, battery, capacitor, etc. The cruise control controller 68 or othervehicle controller (e.g., 54) may be programmed or instructed withcomputer/processor executable instructions stored on a computer-readablemedium to implement cruise-controller acceleration according to two ormore control strategies, which may be based on one of a plurality ofvehicle or cruise control operating modes. The present invention ispredominately described for exemplary and non-limiting purpose withrespect to two control strategies, referred to as a performance strategyand an efficiency strategy, although more strategies having differentdescriptions and purposes may be used. As described in greater detailherein, the use of a particular strategy may not necessarily correspondto the currently selected vehicle operating mode or driver selectedstrategy. For example, the performance strategy may be used in responseto an override event even though the driver has selected an economymode.

The performance strategy, at least in one non-limiting aspect of thepresent invention, is intended to correspond with cruise control basedoperations, e.g., operations controlled by the cruise control system,occurring in a manner that completes a requested acceleration or vehiclespeed with greater performance than the same change would be achievedaccording to the efficiency strategy. The efficiency strategy, at leastin accordance with one non-limiting aspect of the present invention, isintended to correspond with cruise control occurring in a manner thatcompletes a requested acceleration or vehicle speed change moreefficiently than the same change would be achieved according to theperformance strategy manner. One differentiating characteristic of theperformance strategy may be completion of the requested action in lesstime than the action would be completed according to the efficiencystrategy, at least in so far as the efficiency strategy implementingactions that are less responsive and/or require more time for thepurposes of consuming less fuel and/or in a manner that is otherwisemore efficient.

FIG. 2 illustrates a flowchart 80 of a method for economic cruisecontrol in accordance with one non-limiting aspect of the presentinvention. The method contemplates directing cruise control relatedoperations according to one of the performance and efficiency relatedstrategies noted above depending on desired operating parameters of thedriver or an entity otherwise in control of managing the vehicle 10.

Block 82 relates to determining whether cruise control is enabled.Cruise control may be enabled through driver interaction with the cruisecontrol system 68 and/or through remote means, such as if the vehicle ispart of an automated transit system. The present invention contemplatescruise control relating to any automated control strategy where it isdesirable to maintain a constant vehicle speed where the driver, orother entity, may periodically request an increase in the vehicle speedand/or where terrain changes (uphill) or other events may necessitatevehicle acceleration while the vehicle is being controller according tothe cruise control mode. The events requiring the cruise control system68 to take action are collectively are referred to as an accelerationrequest to simplify the exemplary description of the present invention.

Block 84 relates to determining occurrence of an acceleration request.The acceleration request may be determined, for example, when a driverdepresses an acceleration button associated with the cruise controlsystem, the driver verbally requesting acceleration through a vehicleverbal command system (not shown), and/or the cruise control systemidentifying operating conditions where automated control is required tomaintain the current vehicle speed. The request may be defined accordingto any number of parameters, such as net increase in vehicle speed, anet increase in vehicle speed as a function of time, etc.

Block 86 relates to determining which one of the available cruisecontrol strategies has been selected to control meeting the accelerationrequest. The available cruise control strategies may be dependent on thecapabilities of the vehicle, such as whether the vehicle is drivensolely using the ICE, such as if the vehicle does not include the motorand/or energy available for use by the motor is insufficient orunavailable, or using the ICE in combination with the electric motor orother secondary propulsion means. Threshold analysis and othercalculations may be performed to assess the availability of the motorrelative to available energy sources.

FIG. 3 illustrates a graph 90 of operating strategies for the vehicle 10when assistance from the electric motor 40 is unavailable, for example,when the vehicle 10 is operating in ICE only mode. The graph 90represents the performance strategy with a first line 92 and theefficiency strategy with a second line 94. The graph depicts onedecision making process with respect to downshifting the transmission asa function of a current vehicle speed from a current gear to a lowergear in order to meet the requested acceleration demand. Onedifferentiation between the performance and efficiency strategies whenoperating only using the ICE 12 relates to the performance strategypermitting downshifting before a corresponding downshifting would occuraccording to the efficiency strategy, thereby allowing the performancestrategy to be more responsive at the cost of greater fuel consumption,i.e., lower efficiency.

The requested acceleration may be plotted versus the given vehicle speedand relative to one of the first and seconds lines 92, 94 to determinewhether a downshift should occur. A reference value 96 is shown betweenthe first and second lines 92, 94 to demonstrate an operating conditionwhere downshifting would be permitted in the event the performancestrategy was selected and where no downshifting would be permitted inthe event the efficiency strategy was selected. The capability to selectbetween the two strategies to control downshifting may be beneficial inallowing the vehicle 10 to perform more slowly, i.e., complete theacceleration request more slowly, or below its capabilities in order toconserve energy and/or fuel. While the lines are shown to be linear, thepresent invention fully contemplates the lines having other shapesand/or used to control other fuel/energy consuming decision besidesdownshifting.

FIG. 4 illustrate a graph 100 of operating strategies for the vehiclewhen the electric motor 40 is available to assist the ICE 12 in drivingthe vehicle. The graph 100 represents the performance strategy with afirst line 102 and the efficiency strategy with a second line 104. Thegraph 100 depicts a decision making process regarding whether to engagethe ICE 12 when attempting to provide the requested acceleration. Onedifferentiation between these performance and efficiency strategies mayinclude the performance strategy permitting use of the ICE 12 before acorresponding use would occur according to the efficiency strategy,thereby allowing the performance strategy to be more responsive at thecost of greater fuel consumption, i.e., lower efficiency.

The requested acceleration may be plotted versus the given vehicle speedand relative to one of the first and seconds lines 102, 104 to determinewhether ICE only mode should occur. A reference value 106 is shownbetween the first and second lines 102, 104 to shown an operatingcondition where ICE only mode would be permitted in the event theperformance strategy was selected and where motor only mode would beused in the event the efficiency strategy was selected. This capabilityto select between the two strategies to control motor usage may bebeneficial in allowing the vehicle 10 to perform more slowly, i.e.,complete the acceleration request more slowly, or below its capabilitiesin order to conserve energy and/or fuel. While the lines are shown to belinear, the present invention fully contemplates the lines having othershapes and/or used to control other fuel/energy consuming decisionbesides downshifting.

Optionally, in the event the ICE 12 is already engaged in assistingcruise control, such as if the ICE is assisting the electric motor 41,the decision making process shown in FIG. 4 may be used to determinewhether further expenditures required to meet the request accelerationare to occur using the ICE 12 (IC only) or solely using the motor 40(motor only). The ICE only of FIG. 4 may optionally include the ICEcontrol of FIG. 3 in that the downshifting lines may be used todetermine whether a downshift is permitted. This may be beneficial infurther enhancing the driver's ability to select and control cruisecontrol performance according to their desire for greater performance orgreater efficiency.

Returning to FIG. 2, blocks 108, 110 relate to the cruise control systemdetermining powertrain action to be implemented according to one of theperformance and efficiency strategies selected above. Blocks 112, 114relate to the cruise control system 68 issuing instructions to thevehicle controller 54 or otherwise directing the corresponding controlof the powertrain. Block 116 is reached when operating according to theefficiency strategy and relates to a making an assessment of whether toswitch from the currently active efficiency strategy to the performancestrategy upon occurrence of an override event. The override event maycorrespond with an action in response to which it may be desirable tocease operating at the lower performance of the efficiency strategy. Thethresholds and/or conditions used to assess this need may be specifiedby the driver or other controlling entity.

One override event may be determined in the event the driver depresses acruise controller request button for a predefined period of timesufficient to differentiate between a desire to slowly or rapidlyacceleration, e.g., a depression of longer than three seconds may beconsidered as desire to accelerate more rapidly. A similar overrideevent may occur with a particular depression sequence of the button,such as if the button is pulsed or otherwise repeatedly actuated in adiscernable manner understood by the driver to automatically promptselection of the performance strategy. Another override event may bedetermined in the event change acceleration after beginning is slowerthan a predefined threshold, e.g. there may be limit, optionallyspecified by the driver, for how slowly the efficiency mode mayaccelerate before automatically switching to the performance strategy,e.g., change of speed less than one mile per five seconds may be set asthe slowest allowable acceleration before switching to the performancestrategy. Yet another override event may occur in the event vehiclespeed fails to increase after beginning the efficiency controlledacceleration, such as if the motor lacks sufficient power or torque toincrease vehicle speed, which may occur, for example, in the event thevehicle is traveling a sufficiently inclined surface.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the invention.

What is claimed is:
 1. A method of operating a vehicle having first andsecond cruise control modes in which requested acceleration to a targetspeed is greater in the first mode than in the second mode, the methodcomprising: commanding, by a controller, a transition to the firstcruise control mode in response to vehicle acceleration being less thana predetermined threshold for a predetermined period of time during anacceleration request while in the second cruise control mode.
 2. Themethod of claim 1 further comprising commanding, by the controller, anengine to accelerate the vehicle in the first cruise control mode. 3.The method of claim 1 further comprising commanding, by the controller,a traction motor to accelerate the vehicle in the first cruise controlmode.
 4. The method of claim 1 further comprising commanding, by thecontroller, a transmission to downshift while accelerating the vehiclein the first cruise control mode.
 5. The method of claim 1 wherein thefirst cruise control mode is a cruise control performance mode and thesecond cruise control mode is a cruise control economy mode.
 6. Avehicle comprising: a transmission; and at least one controllerprogrammed to inhibit transmission downshifts when operating in a firstcruise control mode and allow transmission downshifts when operating ina second cruise control mode, and in response to vehicle accelerationbeing less than a predetermined threshold for a predetermined period oftime during an acceleration request while in the first cruise controlmode, transition to the second cruise control mode to allow transmissiondownshifts.
 7. The vehicle of claim 6 wherein the transmission isdownshifted in the second cruise control mode to increase vehicleacceleration.
 8. The vehicle of claim 6 further comprising an engine,and wherein the at least one controller is further programmed toaccelerate the vehicle using the engine before downshifting thetransmission in the second cruise control mode.
 9. The vehicle of claim6 further comprising a traction motor, and wherein the at least onecontroller is further programmed to accelerate the vehicle using thetraction motor before downshifting the transmission in the second cruisecontrol mode.
 10. A vehicle comprising: an engine; a traction motor; andat least one controller programmed to accelerate the vehicle using atleast one of the traction motor and the engine in a first cruise controlmode at a faster rate than in a second cruise control mode, and inresponse to vehicle acceleration being less than a predeterminedthreshold for a predetermined period of time during an accelerationrequest while in the second cruise control mode, transition to the firstcruise control mode to increase vehicle acceleration.
 11. The vehicle ofclaim 10 wherein the first cruise control mode is a cruise controlperformance mode and the second cruise control mode is a cruise controleconomy mode.
 12. The vehicle of claim 10 wherein the at least onecontroller is further programmed to transition from the second cruisecontrol mode to the first cruise control mode in response to vehiclespeed not increasing within a predetermined time after the accelerationrequest.
 13. The vehicle of claim 10 further comprising a transmission,and wherein the at least one controller is further programmed todownshift the transmission in the first cruise control mode.
 14. Thevehicle of claim 10 further comprising a transmission, and wherein theat least one controller is further programmed to accelerate the vehicleusing the traction motor before downshifting the transmission in thefirst cruise control mode.
 15. The vehicle of claim 10 furthercomprising a transmission, and wherein the at least one controller isfurther programmed to accelerate the vehicle using the engine beforedownshifting the transmission in the first cruise control mode.
 16. Thevehicle of claim 10 further comprising a transmission, and wherein theat least one controller is further programmed to select between usingthe engine and downshifting the transmission to accelerate the vehiclebased on vehicle speed in the first cruise control mode.
 17. The vehicleof claim 10 further comprising a transmission, and wherein the at leastone controller is further programmed to downshift the transmission toaccelerate the vehicle in the first cruise control mode.